Known tackifiers which are mixed with natural rubber, various block copolymer rubbers or ethylene copolymers, to provide adhesive compositions include hydrocarbon resins obtained by cationic polymerization of a monomer mixture mainly comprising a C.sub.5 fraction. For these adhesive compositions to be used in a step involving heating, means for improving heat stability of the hydrocarbon resins have been proposed as disclosed, e.g., in JP-A-60-104144 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, the conventional hydrocarbon resin tackifiers for adhesive compositions are still unsatisfactory in hue stability.
In recent years, amorphous propylene polymers have found their use in hot-melt adhesives as suggested in JP-A-1-266175, Australian Patent application No. 33060/89 corresponding to JP-A-2-86676 and Australian Patent application No. 63181/90 corresponding to JP-A-3-115483. The hot-melt adhesives comprising amorphous propylene polymers usually contain, as a tackifier, an aliphatic petroleum resin, an aromatic petroleum resin, a terpene resin, rosin or its derivative, a hydrogenated aromatic petroleum resin, a hydrogenated terpene resin, a hydrogenated dicyclopentadiene resin, etc. However, the tolerance of these tackifiers is limited due to poor compatibility between the amorphous propylene polymer and the tackifiers. Further, when heated for a long time, the tackifier tends to separate from the amorphous propylene base polymer to form an oxidized skin which will clog the nozzles of a hot-melt coater. In some cases, a wax or a softener is incorporated into the hot-melt adhesives for the purpose of melt viscosity adjustment. However, since these additives considerably reduce the adhesive strength, the permissive amount of the additives to be incorporated is limited, posing difficulty in obtaining an adhesive well balanced between adhesive strength and melt viscosity.